壳层星系中的GALEX UV发射：追踪星系“复兴”事件
The GALEX UV emission in shell galaxies: tracing galaxy "rejuvenation" episodes
摘要：
我们介绍了三个邻近的壳星系，即NGC 2865，NGC 5018和NGC 7135的GALEX远FUV和近NUV紫外成像。在NGC 2865的NUV图像和NGC 7135的NUV和FUV图像中检测到光学可见的壳和精细结构系统。NGC 5018的NUV图像没有呈现壳层结构。我们检测到的吸收功能，在所有三个星系的核区域。NGC 2865在整个星系中有一个几乎平坦的颜色轮廓，（FUV-NUV）~2。NGC 7135的中心是蓝色的（FUV-NUV）~0，外围是红色的（FUV-NUV）~1.5，包括微弱的贝壳状特征。我们调查的能力，核GALEX（FUV-NUV）的颜色提供信息的再生现象的恒星星族的壳星系。为此，我们从理论上推导出Mg 2，Hbeta，HgammaA，HdeltaA舔线强度指数和（FUV-NUV）颜色之间的关系。我们将研究扩展到在其光谱中具有发射线的早型星系样本（Annibali et al. 2007）。在索引与（FUV-NUV）色图，大多数星系都很好地解释了被动演化的SSP。平均而言，年龄和金属丰度的星系在我们的样品中估计的光学线强度指数是一致的推断（FUV-NUV）的颜色。一般来说，所有的颜色，但（FUV-NUV）和（FUV-V），成为几乎年龄不敏感时，1-2 Gyr已经过去了，从最后一个恒星形成的事件。考虑到复合恒星星族模型与最近爆发的恒星形成，我们认为，在（FUV-NUV）-H β平面的NGC 7135和NGC 2865核的位置可以解释在最近的复兴插曲。（节录）
英文摘要：
We present the GALEX far FUV and near NUV ultraviolet imaging of three nearby shell galaxies, namely NGC 2865, NGC 5018 and NGC 7135. The system of shells and fine structures visible in the optical is detected in the NUV image of NGC 2865 and in both NUV and FUV images of NGC 7135. The NUV image of NGC 5018 does not present shell structures. We detect absorption features in the nuclear region of all three galaxies. NGC 2865 has a nearly flat colour profile with (FUV-NUV)~2 throughout the whole galaxy. NGC 7135 is blue in the center (FUV-NUV)~0 and as red as (FUV-NUV)~1.5 in the outskirts, including the faint shell-like feature. We investigate the ability of the nuclear GALEX (FUV-NUV) colour to provide information about rejuvenation phenomena in the stellar populations of the shell galaxies. To this aim, we derive from theory the relationship between the Mg2, Hbeta, HgammaA, HdeltaA Lick line-strength indices and the (FUV-NUV) colour. We extend the study to a sample of early-type galaxies with emission lines in their optical spectra (Annibali et al. 2007). In the index vs.(FUV-NUV) colour diagrams, most of the galaxies are well explained by passively evolving SSPs. On the average, ages and metallicities of the galaxies in our sample estimated from optical line-strength indices are consistent with those inferred from the (FUV-NUV) colour. In general, all the colours but for (FUV-NUV) and (FUV-V), become nearly age insensitive when 1-2 Gyr have elapsed from the last star forming event. Considering composite stellar population models with a recent burst of star formation, we suggest that the position of the NGC 7135 and NGC 2865 nuclei in the (FUV-NUV)-Hbeta plane could be explained in term of a recent rejuvenation episode. (Abridged)
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